This invention relates to a driving circuit for proudcing a driving signal for a liquid crystal display device (LCD).
A liquid crystal display device is compact, low voltage drivable, and low power consuming and is more and more widely used in place of a cathode ray tube (CRT) in a television receiver of a monochrome or a color television signal, as a monochrome or color monitor for an electronic computer, and in and as a like apparatus. The liquid crystal display device may be either an active matrix one or a more general one.
When driven by a driving signal of a drive voltage, a liquid crystal display pixel of an active matrix liquid crystal display device or a liquid crystal display elements of a more general liquid crystal display device shows a nonlinear luminance versus drive voltage characteristic curve. On the other hand, the cathode ray tube has a luminance versus drive voltage characteristic curve which is specified by a CRT gamma factor or coefficient of 2.2. The gamma factor is defined by a slope of a log-log curve of plots of the drive voltage and a luminance of light emitted by the cathode ray tube.
As a result of analysis, the present inventor has confirmed it possible to approximate the luminance versus drive voltage characteristic curve of the liquid crystal display device by a nonlinear curve which will later be depicted and is generated by a concatenation of first through N-th linear function curves, where N represents an integer which is equal at least to three. Being represented by such a characteristic curve, it is possible to understand that the liquid crystal display device has an LCD gamma factor or coefficient.
For supply to a cathode ray tube, a television video signal is produced by a video transmitter as a transmitter output signal into which a transmitter input signal is subjected to a gamma correction of generating the output signal proportional to about a 0.45-th (1/2-th) power of the input signal. It is therefore necessary to drive the liquid crystal display device of a television receiver by a gamma compensated video signal into which a television video signal is processed in accordance with gamma compensation of compensating for a difference between the CRT and the LCD gamma factors.
For production of the gamma compensated video signal, a liquid crystal display device driving circuit is described in a book which is, when transliterated in accordance with the standard ISO 3602, edited by Terebizyon Gakkai (the Institute of Television Engineers of Japan) under Ookosi-Takanori and published 1985 by Syokodo under the title of "Ekisyo Disupurei (Liquid Crystal Displays)", Chapter 7, Section 5 (pages 221 to 226). Such a driving circuit includes a gamma conversion circuit for converting an input video signal into a gamma compensated video signal in consideration of the LCD gamma factor.
It is possible to use in the driving circuit of the Television Institute (Institute of Television Engineers of Japan) book a digital gamma conversion circuit disclosed in Japanese Patent Prepublication No. 1-220,579. When use is made of such a digital gamma conversion circuit, the driving circuit is herein called a digital type driving circuit. In contrast, an analog type liquid crystal display device driving circuit has been proposed.
In the manner which will later be described in greater detail, the driving circuit of the Television Institute book may comprise a digital gamma conversion circuit which comprises an analog-to-digital converter for converting a input video signal into a digital video signal and a read only memory for converting the digital video signal into a digital gamma compensated video signal. For drive of an active matrix liquid crystal display device, the analog-to-digital converter is put in operation by a clock sequence of pixel clocks of a clock frequency or rate corresponding to the digital video signal and has a converter bit length capable of producing the digital video signal. The read only memory has a memory capacity and memory bit length capable of dealing with the digital video signal and with the digital gamma compensated video signal.
The clock frequency of the converter and the memory bit lengths are very high and long when the input video signal is either a high definition video signal or a color video signal. As a result, the digital type driving circuit has had a defect such that the analog-to-digital converter and consequently the driving circuit is power consuming. Irrespective of the input video signal of such types, the converter and the memory bit lengths must be long if the driving circuit should have a high precision. As a consequence, the driving circuit has had another defect such that the converter and the memory are expensive.
In addition, the driving circuit has been incapable of adjusting gamma compensation continuously for the LCD gamma factor. It may be mentioned here that a liquid crystal display device emits its output light with different luminances depending on a visual angle of watching the display. Consequently, the driving circuit has had a further defect such that it is impossible to achieve continuous adjustment of compensation for the LCD gamma factor and for an LCD gamma factor of an optimum visual angle at which the liquid crystal display device is watched.
The analog type driving circuit comprises an analog gamma conversion circuit for approximating the luminance versus drive voltage characteristic curve for the liquid crystal display device by a polygonal line which is composed of a plurality of line segments generated by line unit generating circuits, each comprising a diode or a like nonlinear circuit element. Limitation has therefore been unavoidable on approximating the LCD gamma factor of a complicated curve. As a result, the analog type driving circuit has had a different defect such that it is impossible to attain a high precision.
In the analog type liquid crystal display, on the other hand, each line unit generating circuit is implemented by a differential amplifier. This is in order to attain a high speed of operation and is different from use of differential amplifiers in this invention in the manner which will later be described.